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The basic apparatus for CE is shown diagrammatically in Fig. 10.17. A small plug of

sample solution (typically 5 30 mm^3 ) is introduced into the anode end of a fused silica

capillary tube containing an appropriate buffer. Sample application is carried out in

one of two ways: by high voltage injection or by pressure injection.

• High voltage injection.With the high voltage switched off, the buffer reservoir at
  the positive electrode is replaced by a reservoir containing the sample, and a plug of
  sample (e.g. 5 30 mm^3 of a 1 mg cm^3 solution) is introduced into the capillary by
  briefly applying high voltage. The sample reservoir is then removed, the buffer
  reservoir replaced, voltage again applied and the separation is then commenced.


• Pressure injection.The capillary is removed from the anodic buffer reservoir and
  inserted through an air-tight seal into the sample solution. A second tube provides
  pressure to the sample solution, which forces the sample into the capillary. The
  capillary is then removed, replaced in the anodic buffer and a voltage applied to
  initiate electrophoresis.

A high voltage (up to 50 kV) is then put across the capillary tube and component

molecules in the injected sample migrate at different rates along the length of the

capillary tube. Electrophoretic migration causes the movement of charged molecules

in solution towards an electrode of opposite charge. Owing to this electrophoretic

migration, positive and negative sample molecules migrate at different rates. However,

although analytes are separated by electrophoretic migration, they are all drawn

towards the cathode by electroendosmosis (Section 10.1). Since this flow is quite

strong, the rate of electroendosmotic flow usually being much greater than the

electrophoretic velocity of the analytes, all ions, regardless of charge sign, and neutral

species are carried towards the cathode. Positively charged molecules reach the cath-

ode first because the combination of electrophoretic migration and electroosmotic flow

causes them to move fastest. As the separated molecules approach the cathode, they

pass through a viewing window where they are detected by an ultraviolet monitor that

Capillary

inlet

Capillary tube Detector

Electrolyte

buffer

Anode Reservoir

HV

Electrolyte

buffer

Reservoir Cathode

Capillary

outlet

Data

acquisition

+ _

Fig. 10.17Diagrammatic representation of a typical capillary electrophoresis apparatus.

429 10.5 Capillary electrophoresis